Ultrasound-based intrusion detectors are well known in the art. Common detectors of this type simply sense and alarm on variations in constant ultrasonic wave fields that are incident thereon. Such variations are generally indicative of a motion within the detector's field of view. When the variation exceeds a preset threshold, the detector issues an alarm.
Simple ultrasound detectors of this type, however, are highly prone to false alarms due, for example, to air turbulence, detector vibration or swaying motion of inanimate objects. The detection threshold may be raised to reduce the false alarm rate, but generally at the expense of reduced detection efficiency. There is, therefore, a recognized need for more sophisticated ultrasound intrusion detectors, capable of discriminating between the actual movement of an intruder and sources of spurious disturbances in ultrasound signals.
U.S. Pat. No. 4,003,045, to Stockdale, which is incorporated herein by reference, describes an intrusion detection system with turbulence discrimination, based on the Doppler effect. Doppler ultrasound signals are received from an area under surveillance. Signals generated due to movement of an intruder in the area are assumed to produce a relatively constant Doppler shift, dependent on the velocity of the intruder's movement, whereas those due to turbulence have a generally random Doppler profile. A threshold is set in the system so as to discriminate against such randomly-shifted signals.
U.S. Pat. Nos. 4,035,798 and 4,319,349, to Hackett, which are incorporated herein by reference, describe intrusion detection systems, based on the Doppler effect, using modulated ultrasound transmitters. In the '798 patent, the transmitter is modulated by keying it on and off at a controlled modulation frequency. In the '349 patent, the frequency of the transmitter is modulated between two frequencies offset above and below a reference frequency. In both patents, Doppler echoes that are received from the detection area of the system are identified by virtue of their being modulated at the same frequency as the transmitted ultrasound radiation. The modulation may be useful in discriminating against spurious Doppler signals due, for example, to a telephone bell ringing in the vicinity of a detector, but not against other types of disturbances, such as Doppler echoes from swaying objects.
A further problem of ultrasound detection systems is that interfering signals may reach the detector from objects outside a distance range of interest. Thus, for example, U.S. Pat. No. 4,910,717, to Terry, which is incorporated herein by reference, describes apparatus for measuring distances to objects using pulsed ultrasound signals. The apparatus includes circuitry for eliminating spurious echoes from objects outside a desired distance range by setting a threshold signal level and by timing the interval between the transmission of an acoustic signal and the receipt of an echo signal from an object, so that echoes from objects outside the range are ignored.
Similarly, U.S. Pat. No. 4,755,973, to Meier et al., which is also incorporated herein by reference, describes a space surveillance system including one or more ultrasonic sources. Each source defines the area of a monitored space, dependent generally on the dimensions and ultrasonic wavelength of the source. A foreign object within the area triggers an alarm, and the system discriminates between objects within the area and those outside. The sources may be driven in either continuous or pulsed mode and at constant or modulated frequency.
U.K. patent application GB 2,115,151A, to Excell et al., which is incorporated herein by reference, describes a Doppler ultrasound movement detection system, for use particularly in safeguarding a selected area that is not bounded by walls. A transmitter sends periodic bursts of ultrasonic signals into the area, and a receiver receives ultrasonic signals reflected from objects in the area. The phases of the transmitted and reflected signals are compared to provide a reference signal. Doppler components in the reference signal are indicative of motion in the detection area, and successive reference signals are compared to control an alarm. The reference signals are gated so that reflections from outside the selected area are disregarded.